When a fluorescent lamp is operating at failure mode or reaches the end of its operating life, the power of the lamp ballast may increase to abnormally high levels. In some cases, the lamp becomes so hot as to cause the glass wall of the lamp to crack. It is thus necessary to employ a protection circuit to protect the rest of the circuit from the over-voltage conditions that may appear under these operating conditions.
It is presently known to protect arrays of parallel-connected fluorescent lamps by circuits which sense an over-voltage condition and interrupt the fluorescent lamps' power supply upon sensing an overvoltage condition that could damage or destroy the lamps.
An example of such a circuit is described in U.S. Pat. No. 4,398,126, in which a thyristor control circuit controls the firing of a thyristor to ground the base electrode of a transistor when an over-voltage condition is sensed. The transistor, when its base is grounded, de-energizes an oscillatory circuit which powers the fluorescent lamps, thus turning them off. This patent also describes a timer circuit which inhibits the firing of the thyristor during the igniting phase of the lamps.
Another similar example can be found in U.S. Pat. No. 5,321,337, in which a base electrode of a transistor which controls the oscillation of the circuit is grounded by a thyristor upon detection of excess voltage. In this circuit, a capacitor is charged by a coupling winding of a protection circuit, which, in turn, triggers a diac that activates the thyristor controlling the base of the transistor. Once the oscillation transistor is in its non-conductive state, it is prevented from oscillating and from supplying excessive voltage to the fluorescent lamps.
A further example of this approach is seen in U.S. Pat. No. 4,928,039, in which a sensing varistor limits over-voltages by charging a capacitor to a negative voltage, which removes base current from a transistor. This stops the oscillation of the transistor, and prevents the fluorescent lamps from being damaged by the over-voltage.
A still further example of this approach is described in U.S. Pat. No. 5,051,661, in which a heat sensing element triggers a thyristor in response to an abnormal voltage or in response to an overheated condition. The heat sensitive element is, in this example a bimetal switch. In the circuit described in this patent, the heat sensitive element triggers the thyristor to render a transistor conductive which, in turn, shorts out the primary winding of a transformer whose secondary windings are coupled to the bases of oscillation transistors. When the oscillation transistors are turned off, the fluorescent lamps are held in an off state.
Another example can be found in U.S. Pat. No. 5,111,114. In this reference, the generation of high amplitude, high frequency voltages which can damage the fluorescent lamps is prevented by turning the oscillation transistors off. This is accomplished by discharging a diac-driving capacitor which triggers one of the oscillating transistors. Once this capacitor is discharged, it is no longer capable of driving the transistor, and prevents the generation of harmful over-voltages and the supply of these over-voltages to the fluorescent lamps.
It is also known to apply a short circuit across a pair of malfunctioning lamps, to thereby prevent their operation. An example of this approach is described in U.S. Pat. No. 4,970,438, in which a varistor causes a capacitor to charge. When the voltage across the capacitor is high enough, it causes the breakdown of a diac which triggers a thyristor. When the thyristor is triggered, an effective short circuit is placed across the varistor and also across the malfunctioning pair of lamps. This prevents excessive power drain from the inverter and damage to the varistor.
As an alternative to shorting out malfunctioning pairs of fluorescent lamps, it is also known to open a switch connecting the fluorescent lamps to their power supply upon detection of an over- voltage using a diode-thyristor combination. This is shown in, for example, U.S. Pat. No. 4,810,936.
Also of interest is U.S. Pat. No. 5,363,017, in which starting capacitors, which shunt the fluorescent lamps upon start-up, are removed from the ballast upon ignition of all fluorescent lamps.
It is also known, as described in U.S. Pat. No. 4,177,403, to limit the igniting current to a low value when the fluorescent lamps fail to ignite. By coupling a temperature sensitive element to an inductor in series with the lamp ballast, the ballast current can be limited, so as to prevent damage to the lamps.
The above-described circuits, however, upon detecting an over-voltage condition, either short out or disconnect the oscillating circuit supplying the fluorescent lamps. This shuts off not only the malfunctioning lamp, but also shuts off a number of other lamps which are not operating at failure mode. This, of course, is a less than optimal condition.
There exists, therefore, a need for a protection circuit for use with a plurality of lamps, which is capable of identifying one or more failing or failed lamp, and capable of shutting down each failing or failed lamp while keeping the rest of the lamps operating at a normal power level.